Doping of semiconductor materials with conductivity-determining type impurities, such as n-type and p-type elements, is used in a variety of applications that require modification of the electrical characteristics of the semiconductor materials. Photolithography is a well-known method for performing such doping of semiconductor materials. To dope a semiconductor material, photolithography requires the use of a mask that is formed and patterned on the semiconductor materials. Ion implantation is performed to implant conductivity-determining type ions into the semiconductor materials. A high-temperature anneal then is performed to cause the impurity dopants to diffuse into the semiconductor materials.
In some applications, such as, for example, solar cells, it is desirable to dope the semiconductor materials in a pattern having very fine lines or features. The most common type of solar cell is configured as a large-area p-n junction made from silicon. In one type of such solar cell 10, illustrated in FIG. 1, a silicon wafer 12 having a light-receiving front side 14 and a back side 16 is provided with a basic doping, wherein the basic doping can be of the n-type or of the p-type. The silicon wafer is further doped at one side (in FIG. 1, front side 14) with a dopant of opposite charge of the basic doping, thus forming a p-n junction 18 within the silicon wafer. Photons from light are absorbed by the light-receiving side 14 of the silicon wafer to the p-n junction where charge carriers, i.e., electrons and holes, are separated and conducted to a conductive contact, thus generating electricity. The solar cell is usually provided with metallic contacts 20, 22 on the light-receiving front side as well as on the back side, respectively, to carry away the electric current produced by the solar cell. The metal contacts on the light-receiving front side pose a challenge in regard to the degree of efficiency of the solar cell because the metal covering of the front side surface causes shading of the effective area of the solar cell. Although it may be desirable to reduce the metal contacts as much as possible to reduce the shading, a metal covering of approximately 10% remains unavoidable since the metallization has to occur in a manner that keeps the electrical losses small. In addition, contact resistance within the silicon adjacent to the electrical contact increases significantly as the size of the metal contact decreases. However, a reduction of the contact resistance is possible by doping the silicon in narrow areas 24 directly adjacent to the metal contacts on the light-receiving front side 14.
FIG. 2 illustrates another common type of solar cell 30. Solar cell 30 also has a silicon wafer 12 having a light-receiving front side 14 and a back side 16 and is provided with a basic doping, wherein the basic doping can be of the n-type or of the p-type. The light-receiving front side 14 has a rough or textured surface that serves as a light trap, preventing absorbed light from being reflected back out of the solar cell. The metal contacts 32 of the solar cell are formed on the back side 16 of the wafer. The silicon wafer is doped at the backside relative to the metal contacts, thus forming p-n junctions 18 within the silicon wafer. Solar cell 30 has an advantage over solar cell 10 in that all of the metal contacts of the cell are on the back side 16. In this regard, there is no shading of the effective area of the solar cell. However, for all contacts to be formed on the back side 16, the doped regions adjacent to the contacts have to be quite narrow.
Phosphorous is commonly used to form n-type regions in semiconductor materials. Both solar cell 10 and solar cell 30 benefit from the use of very fine, narrow phosphorous-doped regions formed within a semiconductor substrate. However, the present-day method of doping described above, that is, photolithography, presents significant drawbacks. For example, while doping of substrates in fine-lined patterns is possible with photolithography, photolithography is an expensive and time consuming process.
Accordingly, it is desirable to provide phosphorous-comprising dopants that can be used in doping processes that result in fine-featured patterns. In addition, it is desirable to provide methods for forming phosphorous-comprising dopants that can be used in doping processes that are time and cost efficient. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.